Method of enameling steel



United States Tatent 6 US. Cl. 29-527 2 Claims ABSTRACT OF THEDISCLOSURE Manufacture of vitreous enameled steel articles from steelcontaining less than 01% carbon and 0.02 to 0.10% phosphorus, includingcold forming articles from such steel, pickling in dilute acid andenameling the articles so formed.

This application is a division of my copending application Ser. No.289,397, filed June 20, 1963, now abanboned.

This invention relates to improvements in the production of porcelainenameled articles.

Heretofore the grades of sheet steel most widely used in the productionof porcelain enameled articles have been (a) for ordinary wares, lowcarbon (.04-.10 carbon) rimmed steel and (b) for more criticalapplications, a lower carbon (.01.03 carbon) low metalloid steel oftentermed enameling iron. With either stock, the conventional enamelingprocedure includes forming the sheet to finished shape, cleaning,pickling and otherwise preparing the surfaces of the formed article andthe application of two coats of enamel, the first termed the groun orblue coat and the second, the color or finish coat.

To reduce enameling costs, there has been a continued effort toeliminate the ground coat from the above sequence. However, pastattempts to apply the finish coat directly to the steel resulted in thedevelopment of surface defects in the coating. Although it was knownthat these defects were caused by the evolution of carbonoxide gasesfrom the steel during the firing of the enamel and could be avoided byfully decarburizing the steel stock prior to the application of theenamel, no commercially feasible method for such decarburization existeduntil the recent development of practical equipment for loose-coiling ofsteel strip. The latter has made possible the modification ofbox-annealing now known as opencoil annealing, wherein, by reason of thefact that the annealing atmosphere has free access to all surfaces ofthe steel coil, the chemical analysis of the steel can be modifiedwithin practical time intervals and in equipment of practical size. Thechanges in steel composition which can be effected by open-coilannealing are controlled by controlling the analysis of the annealinggas, the time and temperature in the annealing operation. Application ofthe open-coil technique using a decarburizing gas in the annealing ofrimmed steel provides enameling stock characterized by a carbon contentof less than 0.008%. This stock has been termed direct-on enamelingsteel since, due to the low carbon content, a single coat defectfreeenamel coating can be produced thereon.

The compositions of conventional enameling stocks are tabulated below:

3,436,808 Patented Apr. 8, 1969 While the above conventional direct-onsteel possesses excellent enameling properties, it suffers from aserious limitation; its extremely low carbon content renders it, whenslightly strained or cold Worked, extremely suscep tible to abnormalgrain growth at the temperatures encountered in firing of the enamelcoating. The result is that any formed areas of the enameled article areextremely weak and therefore easily deformed during subsequent handlingor use. As a consequence, the areas of use of this material have beenlimited.

It is an object of the present invention to provide an improveddirect-on enameling steel which possesses a higher yield strength aftercold working and heating to enameling temperatures than that afforded byconventional direct-on" stock.

Another serious limitation of the conventional directon enameling steelis, it pickles too slowly. The pickling prior to enameling is a criticaloperation in the enameling procedure; experience in the trade has shownthat a definite degree of etch is essential if good coating adherence isto be achieved. 'In the case of the direct-on application of the finishcoat of enamel, i.e. the enameling system in which the use of a groundcoat is eliminated, it has been found in the enameling industry that aminimum of 2 grams of iron per square foot of steel surface must beremoved to properly prepare the surface. The pickling time required toremove this quantity of iron from the currently available direct-onsteel has proven to bebetween 15 and 30 minute-s; in contrast, thepickling times for preparing steels for application of ground-coatenamels is between 3 and a maximum of 10 minutes. This increasedpickling time makes the use of the present direct-on steel economicallyunattractive to enamelers.

Accordingly, it is another object of the invention to provide adirect-on stock which will pickle more rapidly and to accomplish thiswithout serious impairment of either forming or enamelingcharacteristics.

The accomplishment of the aforementioned and other objects will beapparent from the following specification.

In my search for solutions to the above problems, I have discovered thatthe pickling rate of low metalloid steels containing carbon up to about0.008%, is approximately tripled, when phosphorus in excess of about0.02% is added thereto. The effect of phosphorus in this regard isillustrated in the following tabulation:

TABLE II Steel P Weight loss, g./fl;.

10 min.

The above tests steels all contained carbon less than 0.005% togetherwith manganese, sulphur, silicon and other residual elements in theamounts normally present in enameling stock. Steel A is typical of theconventional direct-on enameling steel. The tabulated weight-losses arespecific to pickling the steels for 10 minutes in an 8% sulphuric acidsolution at approximately F. It will be noted that the pickling rate oreffectiveness rises sharply with increasing phosphorus up to about 0.10%but that further increase is relatively ineffective. In applying thisTABLE I C Mn P (max.) S (max.) Si(max.) .Al (max) Fe 1 (total) Rimmedsteel... 0. 04/0. 10 0. 20/0. 50 0. 012 0.035 0. 02 0. 015 Bal.Enameling iron- 0. 01/0. 03 0. 05/0. 20 0. 010 0. 035 0. 02 0. 015 E81.Direct-on 0. 001/0. 008 0. 15/0. 60 0. 012 0. 035 0. 02 0. 015 Bal.

1 May include residual amounts of other elements such as copper, nickel,chromium, molybdenum and tin which do not affect the properties.

discovery, I have found that, under the pickling conditions normallyencountered in commercial enameling operations, a phosphorus content ofabout 0.06% allows removal of the desired minimum of 2 grams of iron persquare foot of steel surface in from 3 to 5 minutes. Such pickling timescompare favorably with those used in preparing the rimmed and theenameling-iron grades of stock.

In addition, I have found that increasing the phosphorus in theseextremely low carbon steel compositions significantly increases theyield strength thereof after critically straining and annealing for 5minutes to simulate enamel firing. The eifect of phosphorus in thisrespect is illustrated below.

4 0.008% and preferably to 0.006% or less. While any suitabledecarburizing atmosphere may be used, I prefer an atmosphere of thefollowing composition:

Hydrogen percent 9 Nzitrogen do 91 Water vapor dewpoint F 55-60Following the annealing operation, the strip is recoiled, temper rolled,if desired, and sheared into cut sheets of ordered length. The resultingsheets are then cold formed into the articles of the desired shape,alkaline cleaned to remove grease and drawing lubricants, rinsed inwater,

TABLE III Minimum yield Phos., Yield Tensile Elong. in Hardness,strength after Steel percent strength, strength, 2", percent Rockwell BSwift cup l enamel firing 1 p.s.i. p.s.i. 5 mm. at

1 The Swift cup test provides a measure of the tormability of sheetmaterials, a description of the Metal Industries, March 1957, p. 203 andApril 1957, p. 257.

st will be found in Sheet 2 Specimens critically strained 4 to 12% tosimulate cold forming prior to enamel firing.

It will be noted that phosphorus up to about 0.10% be added withoutsignificantly affecting the forrnability of these steels and that belowthis amount the essential increase in yield strength after coldstraining and heating is obtained without detectable loss of ductility.As indicated by the mechanical test results the operating range of thephosphorus addition is between about 0.02 and 0.10% and within thisrange I prefer about 0.06%.

The foregoing improvements are obtained without any adverse effect onenameling characteristics, i.e. any steel falling Within the followingcomposition range possesses enameling properties equal or superior tothose of the currently available direct-on stock and are substantiallyfree of the limitations of the latter:

*May include the usual residuals of other elements in amounts which donot deleteriously affect the properties.

In practicing my invention, however, I prefer to manufacture stock ofthe following more limited analysis: 0.006% carbon, 0.15 to 0.45%manganese, 0.03 to 0.06% phosphorus, 0.035% sulphur, 0.01% maximumsilicon, 0.01% maximum total aluminum, with the balance essentiallyiron.

The preferred practice of my invention is outlined below Manufacturestarts with making of the steel. The heat is melted to the usual 0.10%carbon maximum low metalloid rimmed steel specification containing atleast 0.15% manganese using any suitable conventional practice, but thephosphorus content of the heat is adjusted to lie in the range0.02-0.10%, preferably to between about 0.03 and 0.06%. The heat iscast, hot rolled, continuously pickled and cold reduced to strip ofordered gauge and width, in accordance with the conventional practicesused in these operations. Upon completion of the cold reduction, thestrip may be alkaline cleaned to remove cold mill lubricants after whichit is loose-coiled and then open-coil annealed at 1275 to 1300 F. in adecarburizing atmosphere until its carbon content is reduced to at leastcan 30 and then pickled in dilute sulphuric acid (e.g. an 8% by weightaqueous solution of H for up to 10 minutes to remove a minimum of 2grams of iron per square foot of article surface. After rinsing toremove residual pickle acids, the articles are flashed with nickel,using any of the common aqueous nickel salt solutions available for thispurpose. Upon completion of the preparatory steps, the articles aregiven a final rinse preferably in a slightly alkaline neutralizingsolution, dried and subsequently coated with enamel slip. The article isnext subjected to a low temperature drying operation to remove excesswater from the slip and then fired in a suitable furnace for about 5minutes at a temperature sutficient to vitrify the coating. Thetemperature of firing will depend upon the composition of the enamel butwill generally fall in the range of 1300 to 1500 F.

Although the new stock is intended primarily for one coat enameling, itis, of course, admirably adapted to the production of quality ware bythe two-coat process. In the latter case, the first coat of enamelapplied after the preparatory steps will be a ground coat, this is firedand a second coat of slip compounded to the desired color is applied,dried and fired to produce the finished article.

While I have shown and described certain specific embodiments of myinvention, it will be obvious that certain modifications therefrom canbe made without departing from the scope of the appended claims.

I claim:

1. A method of making vitreous enameled steel articles comprisingobtaining steel stock containing 0.001 to 0.008% carbon, at least .15%manganese, up to 015% aluminum and 0.02 to 0.10% phosphorus, coldforming said stock into articles of the desired shape, pickling saidarticles in dilute acid to remove at least 2 grams of iron per squarefoot of article surface, coating said articles with enamel slip and thenheating the coated articles to vitrify the coating.

2. A method of making vitreous enameled steel articles comprisingobtaining steel stock containing .006% maximum carbon, .15 to .45%manganese up to 015% aluminum and .03 to .06% phosphorous, cold formingsaid stock into articles of the desired shape, pickling said articles indilute acid to remove at least 2 grams of iron per square foot ofarticle surface, coating said articles References Cited UNITED STATESPATENTS Kranse 75-123 X Chester 117-53 X Eckel et a1 117-53 Blickwede eta1 117-129 Kopchak 117-129 X Mayer 148-12 6 OTHER REFERENCES Bablik,Galvanizing (Hot Dip), 1950, E. and F. N. Spon. Ld., pp. 62, 63, Ts 660B3-rE, copy in 161.

Cleves et al., The Metal-Iron, McGraw-Hill, 1935, p. 334, copy in 110.

Deringer, Ceramic Industry, Chicago, May 1950 pp. 5659, TP 785C411117-129.

RALPH S. KENDALL, Primary Examiner.

US. Cl. X.R.

